This invention relates to light-emitting semiconductor devices, or light-emitting diodes (LEDs) according to common parlance, and particularly to those monolithically integrated with an overvoltage protector. More particularly, the invention concerns how to improve the light-emitting efficiency of such overvoltage-protected light-emitting semiconductor devices. Overvoltage protectors that can be integrated with light-emitting semiconductor devices according to the invention include, but are not limited to, the Schottky barrier diode, pn-junction diode, varistor, and capacitor.
Japanese Unexamined Patent Publication No. 2000-66863, filed by the assignee of the instant invention, suggests an overvoltage-protected LED of particular pertinence to the present application. It comprises a light-generating semiconductor region of nitride layers on a semiconducting silicon substrate, a current-spreading film of transparent, electrically conducting material on the light-emitting surface of the light-generating semiconductor region, a wire-bonding pad placed centrally on the current-spreading film, a substrate electrode on the underside of the substrate, and an overvoltage protector in or on the substrate. Compactly disposed under the bonding pad, the overvoltage protector hardly adds to the size of the LED.
In the LED of the foregoing known construction, were it not for the current-spreading film under the bonding pad, the current would flow only through that part of the light-generating semiconductor region which lies right under the bonding pad. Covering the entire light-emitting surface of the light-generating semiconductor region, the current-spreading film serves to spread the current flow throughout the light-generating semiconductor region, including its peripheral part away from the bonding pad, and so to cause light emission with a constant intensity from its complete light-emitting surface. The current-spreading film should therefore be as low in electric resistance, and as high in optical transparency, as practical. The material generally favored as meeting these requirements is indium tin oxide (ITO or tin-doped indium oxide), a mixture of, typically, 90 percent indium oxide and 10 percent tin oxide by weight. ITO has its own shortcomings, however.
First of all, ITO is higher in resistivity than metal. It is not totally transparent, moreover, so it had to be fabricated in as thin a film as feasible. Consequently, the ITO film was not sufficiently low in sheet resistance to assure an unvarying current density throughout the light-generating semiconductor region. As far as the applicant is aware, there are no known materials today that are better in both conductivity and transparency than ITO for the current-spreading film.
A solution to this problem is found in Japanese Unexamined Patent Publication No. 2001-237461, which teaches to electrically connect the bonding pad to the current-spreading film via a filamentary connector of radial or latticed pattern on the film. Made from one or more selected metals or alloys, the filamentary connector is less in resistivity, and greater in thickness, than the ITO film. The filamentary connector is therefore capable of better distributing the current flow throughout the light-generating semiconductor region, realizing light emission with a higher efficiency and with a constant intensity from the entire light-emitting surface of the device.
The instant applicant devised a set of connector strips which by themselves are similar to the filamentary connector above, for use in an overvoltage-protected LED. When these connector strips were incorporated in the prior art overvoltage-protected LED referred to earlier herein, the resulting device (shown in FIG. 1 of the drawings attached hereto) proved to possess a serious shortcoming: The connector strips were liable to breakage during its fabrication. This shortcoming had to be overcome by any means in order for the device to win true utility and commercial value.